1. Field of the Invention
The present invention is related to deformable optical devices.
2. Background Art
Light passing through an optical system can become distorted for various reasons. A lens, mirror, coatings thereon, or other devices in the optical system can: have imperfections, contaminants, or defects on their surface or within their structure. These together with thermal and other environmental factors including the ambient properties are sources of error in the light beam. Wavefront aberrations can lead to substantial degrading of the operation of an apparatus having the optical system.
For example, in photolithography where the state of the art requires nanometer level resolution, even small wavefront aberrations in the light beam can cause substantial errors in patterned devices. If these errors are outside of a tolerance budget, the devices will fail. Thus, optical elements within the photolithography systems must be manufactured to exacting tolerances and their environment tightly controlled.
Since practical limits exist in manufacturing tolerances and environmental control, some optical systems use deformable optics, such as deformable mirrors, to help compensate for wavefront aberrations. The deformable mirrors normally include an array of discrete actuators coupled between the mirror and a support. A measuring device (e.g., inline or offline) measures, either continuously or at the beginning of a cycle, the wavefront aberrations at one or more sections of the optical system. A control signal is then generated and transmitted to the actuators, which individually move an area of the deformable optic. The wavefront of the light beam reflecting from the deformed surface is adjusted to compensate for the aberration, and produce a substantially ideal wavefront.
One problem with the conventional deformable optics is that they use rather large actuators to move the optic. Based on the actuator's size and the size of the deformable optic, only a certain number of actuators (e.g., a certain density of actuators) can be coupled to the deformable optic, which limits the amount of fine correction. Density also directly correlates to the type of aberration that can be corrected, i.e., a lower density only allows for correction of lower order (e.g. lower spatial frequency) aberrations. Typical deformable optics can correct for only low order aberrations based on their low actuator density. However, sometimes higher order (e.g. higher spatial frequency) aberrations are necessary to correct. For example, sometimes wavefront aberrations are characterized using Standard Zernike polynomials, including higher orders. Conventional actuator densities cannot adequately correct for higher order terms.
Therefore, a deformable optic is needed that can correct for higher order terms of wavefront aberrations in an optical system.